In recent years electrostimulation therapy according to the so-called interference-current method has become popular, such as described in Austrian Patent No. 165,657. In accordance with this method an alternating current having a medium frequency in a range substantially between 1000 Hz and 100,000 Hz is used, modulated at a frequency of between 0 Hz and 200 Hz. Such a current is passed between two electrodes through the organ to be treated. It is also known to use two pairs of electrodes each passing a respective alternating current through the body, which currents differ from between 0 Hz and 200 Hz so that they beat or heterodyne together in the organ being treated. With such a system the regions immediately adjacent the electrodes are not traversed by enough current for any stimulation effect, but where the two separate currents cross there is an adequate stimulation.
The strength and type of organic reactions to the electrical stimuli are generally determined by the intensity and frequency of the low-frequency amplitude-modulations of the beating alternating currents. So-called vegetative processes are best stimulated by frequencies lying in the lower quarter, that is between 0 Hz and 100 Hz, of the stimulation spectrum. The higher frequencies are more advantageous for tetaniform muscle contraction, these latter being achieved at an optimal frequency of around 50 Hz. Impulse sequences below 25 Hz lead to autogenous vibrations which according to the frequency can cause tension-relieving twitching or a slow and deeply effective kneading.
It is important to note that such electrical stimulation is only effective when the stimulation intensity is varied periodically. When the intensity remains constant its effect drops off due to the accomodation effect and can even result in damage or tiring of the body part being treated. This is overcome by varying the impulses with a frequency of between approximately 0.25 Hz and 0.50 Hz.
With such arrangements it is impossible to produce frequencies which have the exact characteristic needed for proper endogenous stimulation, since the therapeutic effects are often negated by accomodation or tiring of the cells. Furthermore the effect is very limited because the use of two electrical currents which cross each other only gives a very limited, indeed two-dimensional, field of activity. Thus the ions effective to carry out the necessary stimuli all lie in relatively limited planes so that the physiologic and therapeutic effect is greatly limited.
This latter disadvantage has been at least partially overcome in systems such as described in Austrian Patent Nos. 191,082 and 203,157. At least one of the electrodes may be displaced over the person being treated so as to move the region of effectiveness around within him. Also different dosing of the electricity by means of complicated electronic control units can also change the region where the electricity is effective.